(a) Field of the Invention
The present invention relates to an automatic gain control apparatus in a mobile communication system. More particularly, the present invention relates to an automatic gain control apparatus for demodulating downlink signals, including directional beams and beams traveling in all directions that are divided over time, that are transmitted by a smart antenna system in an orthogonal frequency division multiplexing (OFDM) time division duplex (TDD) system.
(b) Description of the Related Art
In recent years, studies on a smart antenna technique, which is a solution for the deterioration of performance of a mobile communication system due to interference signals and channel characteristics, have been actively conducted. Unlike the existing structure in which two diversity antennas are used to combine multipath signals, the smart antenna technique uses an array antenna and a high-performance digital signal processing technique to control an adaptive antenna beam pattern according to a variation in the radio frequency (RF) signal environment to maximize transmitting/receiving performance and capacity. That is, according to the smart antenna technique, unlike the related art in which beams are emitted in all directions, directional beams are emitted to a corresponding subscriber to minimize signal interference of all of the subscribers in a sector, thereby improving communication quality and system channel capacity.
In wireless broadband Internet (WiBro), which is a typical OFDM TDD system, downlink signals used for the smart antenna technique are composed of only the beams traveling in all directions, or directional beams and beams traveling in all directions that are divided over time.
When the downlink signals are composed of only the beams traveling in all directions, a preamble section that is important to perform synchronization is greatly boosted and then transmitted.
Meanwhile, when the downlink signals include user data that travels in all directions and user data that travels directionally, the size of the user data traveling directionally is larger than that of the user data traveling in all directions. The reason is that the beams traveling in all directions are transmitted from one antenna, but the user data transmitted with the directional beams is the sum of signals transmitted from several antennas. Therefore, the size of the user data transmitted with the directional beams is larger than that of the user data transmitted with the beams traveling in all directions by the number of antennas.
An automatic gain control apparatus of a terminal adaptively varies the gains of signals received according to the channel conditions to optimize a demodulating performance.
When the downlink signal is received, first, the automatic gain control apparatus performs variable gain amplification on the received downlink signal and converts the analog signal into digital data. Then, the automatic gain control apparatus uses the converted digital data to calculate the gain, and accumulates the gain during a preamble section to calculate an average value. The automatic gain control apparatus calculates the difference between a reception gain reference value and the reception gain average value during the preamble section, and converts the difference into an analog value. Then, the analog value of the difference is fed back to a variable gain amplifying stage, and is then used to amplify subsequent user data.
In this case, a maximum value of the received signal is set as the reception gain reference value. In a general OFDM system, the automatic gain control apparatus determines the reception gain reference value on the basis of the size of a preamble signal transmitted from the base station. This is because the user data has a variable size, but preamble data has a substantially constant size since a predetermined signal is transmitted.
Meanwhile, during the amplification of the gain of the received signal and the A/D conversion of the amplified signal, whenever the result of the A/D conversion increases one bit by one bit according to the degree of amplification of the gain, a signal-to-noise ratio (SNR) due to a quantization error is improved by 6 dB. On the other hand, when the size of a signal excessively increases due to gain amplification, clipping occurs during an A/D conversion. Therefore, the accurate adjustment of the size of the signal in the variable gain amplifying stage greatly affects the demodulating performance. That is, the automatic gain control apparatus needs to appropriately adjust the gain of a received signal so as to obtain an optimum SNR within the range in which clipping does not occur during an A/D conversion.
Meanwhile, when the downlink signals include both the directional beams and the beams traveling in all directions, the automatic gain control apparatus according to the related art controls the gain of a received signal using only one reception gain reference value corresponding to a preamble signal or the size of the directional beam signal. In this case, when the reception gain reference value is set on the basis of a received signal that is transmitted with the directional beam, a received signal transmitted with the beams traveling in all directions is not sufficiently amplified, resulting in the loss of the SNR. This characteristic may cause serious problems when the terminal is disposed in a place where a channel environment is bad, such as at a cell edge. On the other hand, when the reception gain reference value is set on the basis of the preamble signal, similar to the case in which only the beam signals traveling in all directions are received, the received signal transmitted with the directional beam is excessively amplified, which results in clipping.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.